The Hoatzin looks like a “punk-rock chicken,” and smells like manure because it digests leaves through bacterial fermentation, similar to a cow.
Taxonomy is grappling with DNA and what it is showing in species relationships, especially the evolutionary tree concept and the Hoatzin, in particular.
Some tidbits below.
Birds are the most diverse vertebrates on land, and they have always been central to ideas about the natural world.
DNA research has not solved the mysteries of the hoatzin; it has deepened them. One 2014 analysis suggested that the bird’s closest living relatives are cranes and shorebirds such as gulls and plovers. Another, in 2020, concluded that this clumsy flier is a sister species to a group that includes tiny, hovering hummingbirds and high-speed swifts. “Frankly, there is no one in the world who knows what hoatzins are.” Even the hoatzin’s parasites defied classification: they hosted feather lice found on no other birds.
The hoatzin may be more than a missing piece of the evolutionary puzzle. It may be a sphinx with a riddle that many biologists are reluctant to consider: What if the pattern of evolution is not actually a tree?
Hoatzins—“in some respects the most aberrant of birds,” according to one Victorian ornithologist—were a problem from the beginning. …Even the hoatzin’s parasites defied classification: they hosted feather lice found on no other birds.
Scientists had long assumed, for example, that daytime hunters such as hawks, eagles, and falcons all descended from a single bird of prey. But, in the genetic tree, hawks and eagles shared a branch with vultures, yet falcons turned out to be closer relatives of passerines and parrots. This meant that the peregrine falcon is more closely related to colorful macaws and tiny sparrows than to any hawk or eagle.
Conservative estimates suggest that at least ten per cent of birds hybridize; among South America’s largest group of birds, that number is thirty-eight per cent, according to one recent study. Hybridization may have been rampant in the aftermath of the asteroid strike, when modern-bird lineages first emerged.